A REPRESENTATION OF PASSIVE HINDLIMB POSTURES IN CAT SPINOCEREBELLAR
ACTIVITY.
Bosco, G, Rankin, A. and Poppele, R.
Department of Physiology, University of Minnesota, Minneapolis, MN
55455.APStracts 3:0052N, 1996.
SUMMARY AND CONCLUSIONS
1. We report here about the modulation of DSCT activity by limb posture. In
principle, DSCT activity could represent limb position in one of several ways.
According to a classical notion of DSCT function, DSCT activity might be
expected to correlate with changes in individual joint angles. However, given
the evidence for extensive polysynaptic convergence onto DSCT units, it is
reasonable to propose that DSCT activity represents more global variables such
as the orientation of limb segments or the length and orientation of the whole
limb. 2. In 6 anesthetized cats we recorded the activity of 96 antidromically
identified DSCT neurons while a robot arm passively positioned the left
hindfoot in 20 positions distributed in the sagittal plane, holding each
position for 8 s. For each position, we measured the joint angles, limb
segment angles and the length and orientation of the limb axis (defined as the
line connecting the hip joint to the hindpaw). We used regression statistics
to quantify: a) possible relationships among geometrical variables of the
hindlimb; b) relationships between DSCT firing rate and limb variables. 3.
First, we found a statistically significant relationship among the joint
angles which could be described by a covariance plane accounting for about 70
percent of the total variance. Thus the 3 degrees of freedom represented by
the joint angles in the sagittal plane are effectively reduced to only 2 by
the coupling between joints. This finding resembles that described for the
behaving cat during stance. However, the correlation between the hip and ankle
angles in the passively displaced hindlimb was just opposite to that observed
during active stance. Moreover we observed that the length and the orientation
of the limb axis is determined simply by a linear combination of the three
joint angles. 4. Most of the DSCT neurons (82/96) were significantly modulated
by changes in foot position (one way ANOVA, p<0.001). For those cells, we
explored systematically how their activity was related to limb geometrical
variables. We found mostly linear relationships between individual joint or
limb segments angles and DSCT firing rates. However, while these relationships
were statistically significant, the random variance was often quite high.
Moreover, about 70% of the cells were modulated by more than one joint or limb
segment angle, suggesting that a model incorporating global geometrical
variables might explain a larger fraction of the variance in the neural data.
5. Consequently we tested how well DSCT activity was modulated by the length
and the orientation of the limb axis using a linear regression model with
length and orientation (or the equivalent linear combination of joint angles)
as predictors. We found that this model explained a larger fraction of the
variability in the firing pattern of nearly every modulated cell than did any
of the single joint models tested. 6. We also attempted to account for the
effect of the mechanical joint covariance on this result by accounting for
correlated independent variables in the analysis. We used a regression model
incorporating all three joint or limb segment angles and performed a backward
elimination of insignificant or redundant variables. The result was that 67%
of the neurons were independently modulated by at least 2 joint angles,
indicating that the modulation did not necessarily reflect the biomechanical
constraint of joint angles covariation, but rather a central convergence of
sensory information from more than a single joint. 7. From these results we
conclude that the firing rates of a majority of DSCT neurons encode the
position of the hindfoot relative to the hip joint. We also propose, on the
basis of behavioral studies on the control of cat stance, and also from
previous neurophysiological data, that the representation of hindfoot position
by DSCT neurons could be in the polar coordinate system defined by the limb
axis length and orientation.

Received 6 December 1995; accepted in final form 29 February 1996.
APS Manuscript Number J818-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96